Bug 1844115 - Part 1: Avoid creating String wrapper objects in Iterat…

…or.from. r=allstarschh

Update per <tc39/proposal-iterator-helpers#281>.

Differential Revision: https://phabricator.services.mozilla.com/D183877

UltraBlame original commit: e909fbf5b324f9a1a490c634818c1fea3005f35f

js/src/builtin/Iterator.js

@@ -302,8 +302,27 @@ function
 GetIteratorFlattenable
 (
 obj
+rejectStrings
 )
 {
+assert
+(
+typeof
+rejectStrings
+=
+=
+=
+"
+boolean
+"
+"
+rejectStrings
+is
+a
+boolean
+"
+)
+;
 if
 (
 !
@@ -313,6 +332,21 @@ obj
 )
 )
 {
+if
+(
+rejectStrings
+|
+|
+typeof
+obj
+!
+=
+=
+"
+string
+"
+)
+{
 ThrowTypeError
 (
 JSMSG_OBJECT_REQUIRED
@@ -331,6 +365,7 @@ obj
 )
 ;
 }
+}
 var
 method
 =
@@ -408,32 +443,13 @@ IteratorFrom
 O
 )
 {
-if
-(
-typeof
-O
-=
-=
-=
-"
-string
-"
-)
-{
-O
-=
-ToObject
-(
-O
-)
-;
-}
 var
 iterator
 =
 GetIteratorFlattenable
 (
 O
+false
 )
 ;
 var
@@ -1669,6 +1685,7 @@ innerIterator
 GetIteratorFlattenable
 (
 mapped
+true
 )
 ;
 var